This invention relates to a vehicle propulsion system having a continuously variable ratio transmission and, more particularly to a control system and a control method for such a propulsion system.
The quest for greater fuel economy of automotive vehicles has led to significant improvements in engine and transmission design and control. Continuously variable ratio transmissions (CVT) have shown particular promise in this regard. It will be appreciated that at any given vehicle speed, and for any need propulsive force, a certain transmission ratio will provide maximum fuel economy for a given engine. In addition, for any given vehicle speed, one transmission ratio will permit maximum acceleration with that engine. Since a CVT with the proper ratio range can provide any desired transmission ratio, it is obviously attractive for automobiles from the standpoint of economy, low emissions and performance. If the mechanical efficiency of the CVT is high and its ratio range is wide enough, it can even be possible to have both maximum economy and maximum performance in the same vehicle. Among the obvious benefits are fully automatic operation, smooth, stepless and rapid response to driver demand, and quieter crusing.
Many different CVT configurations have been developed in the prior art. These include, for example, hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multi-speed gear boxes with slipping clutch; and V-belt traction drives. Of these the V-belt traction drives appear attractive for small to medium size passenger car applications because of their compactness, lightness and simplicity of design. Basically, this type of CVT comprises a V-belt which interconnects a driver sheave and driven sheave, the diameters of the sheaves being variable to change the ratio of the CVT. Recent advances in belt design have resulted in improved belt durability and longevity. If sheave movement can be properly controlled so as to avoid undue stress on the belt, it is expected that a very long belt life can be achieved.
Many control schemes have been devised for engine-CVT systems in attempts to maximize fuel economy. These have been based on empirical analyses of individual engine performance, and the realization that, for any desired power output, there is an optimum combination of engine speed and torque which will result in minimum fuel consumption. This is illustrated in FIG. 1.
FIG. 1 is a typical performance map of a four cylinder spark ignition passenger car engine having a displacement of approximately 2.5 liters. The map is a plot of engine torque T.sub.E and brake horsepower BHP as a function of engine speed N.sub.E. The dot-dash line near the top of the map is a plot of engine torque at full throttle. The series of curves in solid black lines are fuel consumption contours, indicating constant brake specific fuel consumption (BSFC) in 1 b.M/BHP-hr. Minimum fuel consumption occurs at a point designated by 0.4 pounds per horsepower-hour. The series of dashed lines indicates power output of the engine. The ideal operating line for low fuel consumption is indicated by the heavy solid line f(N.sub.E), this curve being a function of engine speed. The ideal operating line for low fuel consumption is purely a function of engine characteristics and is optimal regardless of vehicle road speed. Other ideal operating lines may appear on the performance map, for example, the ideal operating line for low emissions.
In a vehicle with a conventional, manually shifted gearbox, forward speed ratios usually are available in only four or five steps. The operating point of the engine on the performance map is determined by drive shaft speed, power or torque commanded, and transmission gear ratio. Since there are only a few gear ratios available in a typical transmission, the engine must be throttled much of the time. The engine must therefore operate most of the time at high BSFC values. In contrast, a CVT is able to vary its speed ratio continuously to allow the engine to run at wider throttle and lower BSFC values.
Perhaps the most difficult task demanded of a control system for an engine-CVT system is to maintain engine operation along the ideal operating line. This is due to the almost continuous transient nature of operation of an automotive vehicle, there being hardly ever a time when road load and commanded torque or power remain constant. Transient conditions usually are dealt with by a change in CVT ratio, engine speed and throttle. Prior art control systems, by their very nature, permit an excursion of engine operation away from the ideal operating line before returning back to it at steady state. An example of such an excursion is shown in FIG. 1 by dashed line X-Y-Z. The result is that engine operation approaches, but hardly ever is maintained on the ideal operating line.
In virtually all prior art engine-CVT control systems, throttle position is controlled directly by the vehicle accelerator pedal, or is a direct function of pedal position, as well as other parameters. Engine and transmission control usually are directly related to one another. Such control schemes permit engine operation during transients to vary from the ideal operating line. Excursions away from the ideal operating line result in less than optimum engine operation (e.g., excessive fuel consumption, or excessive emissions), until effective control is resumed by the system during steady state operation. As pointed out earlier, however, most vehicular operation is transient in nature, rather than steady state, so that substantially all engine operation occurs off the ideal operating lne. Emissions calibrations must therefore be made in a substantial portion of the engine performance map. Most prior art control systems also must be specifically tailored to particular engines. This requires numberous specifically designed control systems for a fleet of differently powered vehicles. In addition, most prior art control systems cannot compensate for varying engine conditions, the result being vehicle driveability which varies with engine temperature, state of tune, age and altitude. Close duplication of conventional vehicle characteristics also is a problem with prior art CVT control schemes.
Commonly assigned and copending U.S. application Ser. No. 380,922, filed May 21, 1982 (now U.S. Pat. No. 4,459,878)--which is incorporated herein by reference--discloses an elegant solution to the above-noted problems inherent in prior art engine-CVT control systems. Briefly, the control scheme disclosed therein involves totally independent engine and transmission control. That is, the position of the engine throttle is totally independent of accelerator pedal position. Throttle position and, hence, engine output torque simply is a function of engine speed only, and that function may be any desired relationship, for example, the ideal operating line for low fuel consumption, the ideal operating line for low emissions, or a compromise ideal operating line for low fuel comsumption and low emissions. Torque, power or other desired performance parameters commanded by the accelerator pedal controls CVT ratio, and engine speed is determined by the load placed thereon, which is a function of road load and CVT ratio. Hence, throttle position is precisely adjusted in accordance with the ideal function for any load placed on the engine. With appropriately designed controls, anomalous engine and vehicle behavior, such as engine overspeed and underspeed conditions, can be prevented, transient start-up from rest can be accommodated, and the vehicle can be made to perform almost in all respects just as a vehicle with a conventional automatic transmission. This control scheme is described below in greater detail.
It has been found that, due to inherent engine characteristics, the driveability and control of a vehicle governed by such a control scheme may be less than optimum. That is, the "feel" of the vehicle at certain engine speeds in response to accelerator pedal inputs may not closely enough approximate the feel of a vehicle with a conventional automatic transmission. Specifically, unless accelerator pedal movements are smoothly and meticulously controlled by the driver at these engine speeds, the vehicle may tend to buck and lurch, and the engine may tend to speed up too quickly upon acceleration and run to fast.
The problem is caused by the relatively steep nature of the engine torque-speed characteristic or curve of most engines, close to or along which engine operation desirably takes place. The problem tends to be prevalent in the case of many normally aspirated spark-ignition internal combustion engines, which inherently have relatively steep torque-speed characteristics at low engine speeds. An example of this is illustrated by the ideal operating line f(N.sub.E) in the plot of FIG. 1. In the example shown, a significant proportion of engine operation occurs at speeds below 1600 rpm, especially in the case of urban driving at low to moderate road speeds. In this "critical" engine speed range, a slight deviation in engine speed results in a large change in engine torque, meaning that vehicle movement is very sensitive to accelerator pedal input. This sensitivity is heightened at higher transmission ratios--such as during start-up from rest--because engine torque is multiplied to a greater degree before reaching the driving wheels of the vehicle.